Various types of plasmas are produced using a wide variety of methods and are used, for example, for the modification of substrates. Plasma treatment is used, for example, for coating, cleaning and etching substrates, in medicine for the treatment of implants and in technology for purifying waste gases. The geometry of the workpieces to be treated ranges from flat substrates, fibre bundles or materials in web form, to any configuration of shaped articles. High efficiency and ready availability of microwave generators have given microwave plasmas considerable importance.
A variety of types of plasma treatment apparatus are known. In them, microwaves are injected into a plasma chamber via a feed and, optionally, a coupler. Different plasmas are used for various applications. Amongst other things, waveguides and coaxial cables are used for feeding the microwaves, and amongst other things antennas and slots are used for the coupling (DE 4235914).
A wide variety of different types of devices are used for the production of microwave plasmas. According to the prior art, the devices contain a plasma chamber, a container or working space contained therein, and a feeding waveguide which can be coupled thereto, often designed in the form of a surrounding waveguide resonator. It is expedient for these to be arranged on a common axis of rotation. Through input coupling, the microwaves emerge from the resonator into the plasma chamber (FR-A2 26 68 676, EP-A1 0 593 931). However, it has been shown that the known type of input coupling, for example through slots parallel to the axis of rotation and the design of the resonator, is not advantageous. A disadvantageous effect in this case is that, depending on the arrangement of the coupling points, it is not possible to excite particular field patterns, for example TM01n modes. This rules out important applications. There is therefore a need to provide a device for the uniform excitation of as many modes as possible and/or for the production of a large-scale particularly uniform plasma, the device operating in a wide pressure range of from 10.sup.-5 mbar to, in particular, more than 1 bar.